Method and apparatus for low power, regulated output in battery powered electrotherapy devices

ABSTRACT

A device for controlling the discharge of a battery supplying an intermittent load, such as a nerve stimulation device. The device includes a controller which operates a switched inductor to feed a capacitor with numerous small pulses from the battery, thereby building up the charge and voltage on the capacitor, and occasionally discharging the capacitor to a load. The capacitor discharge is at higher current than the small pulses from the battery, so the battery is drained at small instantaneous discharge rates compared the too high instantaneous discharge current from the capacitor.

This application is a continuation of U.S. application Ser. No.09/592,706, filed Jun. 13, 2000, now abandoned, which was a continuationof U.S. application Ser. No. 09/148,837, filed Sep. 4, 1998, now U.S.Pat. No. 6,076,018.

FIELD OF THE INVENTION

The present invention relates generally to the field of battery powereddevices. More specifically, the present invention relates to batterypowered electrotherapy devices such as non-invasive nerve stimulationdevices, Transcutaneous Electrical Nerve Stimulator (TENS) devices,muscle stimulators, etc.

BACKGROUND OF THE INVENTION

Portable electrotherapy devices typically utilize a relatively smallbattery to power internal circuitry which, in turn, provides an outputin the form of an electrical signal. The electrical signal output ofsuch devices has been shown to have therapeutic benefit to a patient fora wide variety of medical conditions. These devices have been describedas non-invasive nerve stimulation devices, electro-acupuncture devices,and in acupuncture terminology stimulate an acupuncture point.

Portable electrotherapy devices are most conveniently powered by coincell or button cell batteries such as those used for hearing aids,calculators and other small consumer electronic devices. Through pulsegenerating circuits, these devices may deliver thousands of pulses perhour from the battery. For example, an electrical pulse repetition rateof approximately 70 pulses per second and a pulse width of 80microseconds have been found to provide effective relief of nausea.Hundreds of thousands to millions of pulses may be required to treatlong lasting nausea, as may be required for post operative nausea,chemo-therapy nausea and other long lasting nausea conditions.

These electrotherapy devices provide a variety of pulse amplitudevariations and combinations, or bursts of pulses at specific intervals.For example, the electrical pulse pattern used in our ReliefBand®electrotherapy product comprises about 350 microsecond pulse width atabout 31 pulses per second at power levels of about 10–35 milliamps peakpulse height. A wide range of pulse patterns may be used in noninvasivenerve stimulation devices.

Other electrotherapy devices are designed for Functional ElectricalStimulation (FES), which exercises muscles near the point ofstimulation. Transcutaneous Electrical Nerve Stimulation (TENS) inhibitssensory nerve communications in the area of stimulation to block pain.All these battery powered electrotherapy devices are characterized by astimulation output, typically in the form of a voltage or current pulsedelivered at a particular pulse shape and waveform. The pulse amplitude,pulse width, and pulse frequency are selected so as to be suitable fortreating particular symptoms or conditions such as pain, addiction,nerve disorder, muscle disorder, organ malfunction, etc. Patients usingthese devices receive direct benefit through the improvement of theirquality of life.

The energy needed to deliver the stimulation output is delivered from abattery supply, which may consist of one or more battery cells at aparticular nominal voltage and particular battery chemistry. OurReliefBand® electrotherapy device uses coin cell batteries of standardsize, which are readily available. The stimulation output peak pulseamplitude is commonly in a range of 1 to 100 milliamps, which isdelivered for a particular time depending on the pulse waveform.

Many types of batteries suitable for use in battery poweredelectrotherapy devices are optimized to deliver electrical current atlower loads than the required stimulation output. For example, a typicalcoin cell battery may be rated to provide 0.1 to 0.3 milliamps ofcurrent for 100 minutes if the battery is drawn down at an averageelectrical current draw of 0.1 to 0.3 milliamps.

As a result of the discrepancy between the optimal current draw on thebattery and the current draw required for therapeutic pulses, thebattery is not used optimally and battery performance and battery lifeare degraded. Because of battery chemistry, the overall amount of powerthat can be drawn from a battery is smaller for large current drainsthan for small current drains. A battery may be able to provide 0.02milliamps for 100 minutes, but may only provide current of 0.1 milliampsfor 10 minutes (instead of 20 minutes), so that half the battery poweris lost if the current is drawn off rapidly. Moreover, the problembecomes even greater as the current draw is increased. Thus, drawingcurrent at the rate of 1 milliamp will not provide the expected 1 minuteof current (at an expected half power loss), but will provide far less,perhaps only a small fraction of a minute of current at 1 milliamp. Forexample, if a battery is discharged for a 10 millisecond pulse of 1milliamp every second, the average current draw is 0.01 milliamps, butthe battery will be depleted according to the instantaneous current of 1milliamp, not the average current of 0.01 milliamps. Rather thanobtaining 100 minutes of operation, the battery will provide far lesscurrent and power. If, however, the battery is discharged at 0.02milliamps for a 0.5 second pulse every second, the average current drawstill is 0.01 milliamps, but the battery will last according to theinstantaneous current draw of 0.02 milliamps. The battery will provide100 minutes of current when drawn down in this manner.

Battery powered electrotherapy devices usually require a higher voltagetherapeutic output pulses than can be provided by conveniently availablebatteries. Accordingly, electrotherapy devices typically use atransformer to step up the pulses from the battery output to the highervoltage output required for therapeutic devices. The high voltage outputis required to allow the pulsed electrical current to be delivered to aparticular electrical load, for example, living tissue. The electricalimpedance of human skin can be modeled as a 500 ohm resistance.Accordingly, if the device is to deliver 30 milliamps into the skin,then it needs to provide a 15 volt output across the skin.

In a conventional electrotherapy device, a transformer is typicallyconnected to the battery, either directly or through a switchingmechanism, and the voltage output from the transformer is proportionalto the battery voltage. A problem occurs when the battery voltage beginsto lower as the battery becomes depleted. Because the high voltageoutput is proportional to the battery voltage, the output voltagecapability lowers and eventually the electrical current output alsolowers for a given electrical load. When the current output lowers, thedevice's therapeutic effectiveness is lessened.

This problem is a serious problem for patients who use electrotherapydevices for chronic conditions. The patient may experience a lowerquality of life, and possibly a degradation in health, as the device'stherapeutic effectiveness diminishes over time. The device may provide alow battery indicator, but effectiveness may still be diminished. Thedevice may also just shut off if the battery becomes too depleted, atwhich point the individual is left without the therapeutic benefit ofthe device with no adequate warning to allow for a replacement device orbattery supply to be obtained. Moreover, current electrotherapy devicesdo not manage battery consumption so as to obtain the maximum availableamount of power from the battery. This leads to more frequent batteryreplacements than would be required if the battery power could bemanaged more effectively.

Various circuits have been proposed for use in monitoring chargeremaining on a battery, or to generate a pulse from a battery for use inan electrotherapy device. A number of devices have used methods formeasuring remaining battery capacity directly for implanted devicese.g., Renirie et al., U.S. Pat. No. 5,369,364, Schmidt, U.S. Pat. No.5,369,364, Arai, U.S. Pat. No. 5,744,393, Thompson, U.S. Pat. No.5,391,193. These methods may include switching to an alternative powersource e.g., Fischell, U.S. Pat. No. 4,096,866, or disabling thetherapeutic output on a low battery condition, e.g., Putzke, U.S. Pat.No. 4,024,875, but do not address the regulation of the stimulationoutput as the battery is depleted.

Privas, U.S. Pat. No. 5,218,960, describes a low battery voltagedetector for stopping stimulation pulse generation when the battery istoo low, but that method requires a priori knowledge of the low batterycutoff voltage so that the circuit component values can be setaccordingly. Privas does not address the problem of the therapeuticoutput voltage lowering as the battery voltage lowers to the cutoffvalue, thereby decreasing the therapeutic value of the output. Also,Privas does not provide the individual with adequate warning of thepending low voltage condition and cessation of therapeutic output,rather, the output is stopped and the low battery signal is given at thesame time.

Dufresne et al., U.S. Pat. No. 4,926,864, describes a circuit forgenerating a high voltage and monitoring that high voltage throughcircuit feedback to maintain the high voltage within a specified rangeas the battery supply is depleted. The Dufresne et al. method suffers inthat the charging pulse width in the high voltage generator must belengthened as the battery supply is depleted. This causes an increase inpower consumption that Dufresne et al. address by limiting the chargingpulse width to a maximum value. As a consequence, the Dufresne et al.method cannot dynamically adjust the monitored high voltage generator totake advantage of the full range of battery supply capacity. Further,Dufresne et al. makes no provision for adequately warning the patient ofthe remaining battery life when their control circuit switches to alengthened pulse width.

Owens, U.S. Pat. No. 5,065,083, describes a system for monitoring thebattery voltage and decreasing the output power to allow the system tooperate at lower battery voltage as battery power decreases duringnormal use. The Owens method suffers in that output power must bedecreased, rather than maintained at a constant, therapeutic level.Although Owens provides for a low battery indicator, the only indicationgiven is that the output has been decreased. It does not provide for anyindication of remaining battery life.

SUMMARY OF THE INVENTION

The battery discharge circuit of the present invention is designed toenhance the battery life of battery powered electrotherapy devices. Anelectronic pulse generator limits peak current draw from a batterysupply so as to extend the battery life. It maintains a constanttherapeutic pulse output to the patient as the battery supply isdepleted, even as the battery output voltage declines. The pulsegenerator accomplishes this by discharging the battery into anintermediate storage device at optimal discharge rate, storing thecurrent in this device and periodically discharging the stored currentin a high current, short pulse width therapeutic pulse output. So far asthe battery is concerned, it sees a peak current draw in its optimalrange, but so far as the patient is concerned, the patient feels atherapeutic pulse that far exceeds the optimal current draw for thebattery. The therapeutic output pulse may also be converted to a voltageseveral times higher than the battery voltage. The circuit may alsoprovide the patient with a low battery warning with adequate time toobtain a replacement device or replacement battery source whilemaintaining a consistent therapeutic output pulse to the patient.

The battery discharging circuit is described in connection with its usein an electrotherapy device. However, the method of discharging thebattery at low average current by pulse charging a capacitive storageunit may be employed in various other environments where high currentintermittent loads are powered by a battery. For example, flashingsafety lights, intermittently operating electrical motors such as thoseused on power tools, battery operated defibrillators, etc. may bepowered by batteries through the circuitry described below to obtainextended battery life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a pulse diagram for an electrotherapy device pulse generatorwithout a controlled battery draw.

FIG. 1 b is a pulse diagram for an electrotherapy device pulse generatorwith a controlled battery draw.

FIG. 2 is a block diagram of a pulse generator with a controlled batterydraw.

FIG. 3 is a circuit diagram of a pulse generator with a controlledbattery draw.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a pulse diagram for an electrotherapy device pulse generatorwithout a controlled battery draw. Here, a battery is discharged onceduring each pulse period and at a high amplitude for each correspondingtherapeutic pulse sent to the patient. As shown by battery dischargeline 1, the pulse amplitude exceeds optimal peak current for thebattery. The battery voltage, indicated by the battery voltage line 1 a,declines steadily as the battery is depleted. The number of pulses thatmay be obtained in this system is small compared to the number of pulsesthat may be obtained with pulses at the optimal current. By contrast,FIG. 1 b is a pulse diagram for an electrotherapy device pulse generatorwith a controlled battery draw. The pulse generator limits the currentdrawn on the electrotherapy device's battery supply, thus preserving thelife of the battery, by discharging the battery in multiple smallamplitude pulses. As shown by battery discharge line 2, many dischargesat the optimal current draw are created in the pulse period p. Thesebattery discharges are sent to and stored in a switched inductor that inturn discharges pulses to a storage capacitor with pulses indicated byinductor output line 3. The current from the inductor is sent to thecapacitor, and charges the capacitor according to the capacitor voltage(or current) status line 4. The storage capacitor stores the switchedinductor discharge pulses until commanded to discharge to create atherapeutic pulse. When the capacitor is discharged, it creates a pulsedoutput pulse indicated by capacitor output (discharge) line 5. Thevoltage output of the capacitor may be stepped up by a transformer, forexample, to provide the desired output level of the therapeutic pulses.The transformer output is shown at line 5 a. The pulse discharged fromthe storage capacitor is the desired battery current discharge amplitudecreated by an accumulation of optimal current discharge pulses from thebattery. The capacitor discharge pulses are maintained at constantvoltage, even as the battery voltage drops, as indicated by batteryvoltage line 2 a. The battery voltage also declines at a much lower ratecompared the direct discharge system illustrated in FIG. 1 a.

Essentially, instead of drawing one large pulse out of the battery whena therapeutic pulse is needed to be sent to the patient, theelectrotherapy device pulse generator characterized in FIG. 1 b draws aseries of smaller pulses, each of optimal pulse amplitude, whichaccumulate and are stored to be discharged when a therapeutic pulse isneeded to be sent to the patient.

FIG. 2 is a block diagram of the electrotherapy device pulse generatorwhich accomplishes controlled battery draw as illustrated in FIG. 1 b.Control module 6 connects the switched inductor 7 to the battery supply8, generating a current draw from the battery supply 8 through theswitched inductor 7 and into the low voltage storage capacitor 9. Thecurrent through the switched inductor 7 rises gradually and a voltagedevelops across the low voltage storage capacitor 9 due to the storageof electric charge from the current. Control 6 then disconnects theswitched inductor 7 from the battery supply 8. This is called a batterydischarge pulse. The switched inductor 7 then releases residualelectrical current into the low voltage storage capacitor 9 causingslightly more voltage to develop across it. The low voltage storagecapacitor 9 is then in an open circuit condition and holds theaccumulated voltage. Repetitively causing electric current to flowthrough the switched inductor 7 into the storage capacitor 9 causes avoltage to build up across the storage capacitor 9 in a step-wisefashion. The capacitor functions as an intermediate storage device, andmay be replaced with other forms of storage devices.

The control 6 charges the capacitor 9 until it has reached the desiredintermediate output pulse voltage, and then causes the capacitor 9 todischarge into the output stage 10. To accomplish this, the control 6must monitor the capacitor voltage. Control 6 monitors the voltage builtup across the low voltage storage capacitor 9 through the voltagedivider switching network 11 connected to the voltage comparator 12.Control 6 continues to operate the switched inductor 7 to build up astore of charge or current in the low voltage storage capacitor 9 untilthe voltage comparator 12 signals to control 6 that the correspondingvoltage across the low voltage storage capacitor 9 has reached apredetermined voltage value. The voltage on the low voltage storagecapacitor 9 is then delivered to the output stage 10. The output stage10 steps up the voltage and delivers it to electrodes 13, therebygenerating a therapeutic pulse through the electrodes to the patient.Various output stages may be employed. For example, where the dischargeis intended for direct output to a load, the output stage may simplyconsist of an electrical terminal.

The predetermined voltage value is that value that has been determinedto provide the optimal therapeutic pulse to the patient, and mayencompass several different values. It is determined from a large numberof observations and tests on patients, which indicate an optimal rangeof output voltages at the electrodes and a corresponding output voltagefrom the low voltage storage capacitor. It is measured in part based onthe voltage divider switching network 11. The particular predeterminedvoltage and the voltage division used by the system is varied by thepatient's manipulation of a switching mechanism (shown and described inFIG. 3). Thus, the predetermined voltage may be predetermined by thevalues of circuit components used in building the circuit, theprogramming of the control module, and the user's own adjustment of thedevice.

Control 6 limits the battery discharge pulses and time between batterydischarge pulses such that the resulting average electrical current drawfrom the battery supply is within the battery's optimal discharge rate,i.e., low enough so as not to degrade battery performance and batterylife. Narrow battery discharge pulses are preferred to limit theinstantaneous electrical current draw from the battery and to be able touse small, low cost inductor components. However, a narrow batterydischarge pulse requires a fast operation of the control 6, whichincreases power consumption the faster it is operated. The choice ofinductor also determines the maximum battery discharge pulse width inorder to avoid saturation of the inductor. The control 6 also needs tooperate at a minimum speed in order to be able to accomplish all of itsfunctions and still be able to deliver therapeutic pulses at the desiredrate. A balance is typically found empirically between these variousfactors. We have found that a battery discharge pulse of approximately 4microseconds delivered at a frequency of approximately 19.5 kilohertz isfavored for 3 volt lithium/MnO₂ coin cell batteries used in thepreferred embodiment. Other factors contribute to the electrical drawand these must be carefully considered. For example, indicators shouldbe chosen for low electrical current requirements and should be pulsedso that their average current draw is minimized.

Control 6 also sets the battery discharge pulse width and time betweenbattery discharge pulses so that there is sufficient time for thetherapeutic pulses to be generated at the required frequency to thepatient. Control 6 does this by counting the number of battery dischargepulses needed to achieve the predetermined voltage on the low voltagestorage capacitor (charge pulse count), such that as the battery supplyis depleted, more battery discharge pulses are ordered to be sent to thelow voltage storage capacitor. The therapeutic pulses are therebymaintained while the battery supply 8 is depleted. Battery dischargepulses can be counted for each therapeutic pulse delivered.Alternatively, control could periodically attempt to charge the lowvoltage storage capacitor up to a maximum predetermined or arbitraryvoltage as part of a calibration routine to estimate the state of thebattery at predetermined intervals. This method obviates the need forthe voltage divider switching network, which can then be replaced with asimple voltage divider. The pulse count can be used by a softwarealgorithm stored in control 6 to calculate the number of pulses neededfor any intermediate voltage, for example, to achieve automatictherapeutic pulse amplitude modulation. As the battery voltage declineswith use, the number of charging pulses needed to achieve a particularcapacitor voltage will increase, so the software algorithm must be ableto accommodate this change, for example, by using different equationsfor different battery voltage ranges or by using various look-up tablesfor different battery voltage ranges. It is also possible to eliminatethe need for a software algorithm through the exclusive use of look-uptables stored in additional program storage space in control 6.

When the control determines that the battery has reached a predeterminedlow battery value (by tracking the charge pulse count or otherwise), thecontrol 6 changes the dual indicator 14 from a normal mode indicator toa low battery indicator and continues to deliver therapeutic pulses. Thelow battery value is calculated as a percentage of the total time thatcontrol can maintain the therapeutic pulses on average for the type ofbattery supply used. For example, if the battery supply allows controlto maintain the therapeutic pulses for an average of 100 hours, the lowbattery value could be set at 80%, leaving the patient 20 hours ofcontinued treatment and sufficient time to get a replacement device orbattery supply.

Referring again to FIG. 1 b, the capacitor charge status line indicatesthat, after many pulses, the battery voltage drops and more optimalpulses must be initiated to charge the capacitor to the output voltage.Eventually, the battery will discharge to a point that, no matter howmany discharge pulses are initiated within the pulse period p, thebattery cannot charge the capacitor to the output voltage. Thus, thevoltage comparator never sees an adequate voltage, and the systemcontroller will not initiate an output pulse from the capacitor to thetransformer. Once control determines that it is unable to regulate theoutput due to a depleted battery supply, it stops generating therapeuticpulses to prevent a degradation of the therapeutic benefit from thedevice. This could be accomplished in a number of ways. Control 6 cancontinue to charge the low voltage storage capacitor, which will nolonger output pulses to the output stage since the voltage comparatorwill never signal to control that the voltage across the low voltagestorage capacitor has reached the predetermined voltage value. Control 6can also repeatedly and rapidly discharge the low voltage storagecapacitor 9 into the output stage 10 in order to rapidly deplete thebattery 8 to the point where it cannot sustain any function. The control6 may also continuously close Q1, leaving the battery 8 continuouslyconnected to the switched inductor 7. Alternatively, control 6 canswitch to a back up battery supply automatically to continue generationof therapeutic pulses powered by the backup battery. Furthermore,control 6 can indicate to the patient that therapeutic pulses are nolonger being delivered by either turning off the indicator lights orlighting a third indicator light.

FIG. 3 is a detailed circuit diagram of an electrotherapy device pulsecontroller. The circuit 20 is powered by a battery B1. The battery isselected on the basis of its battery capacity rating, which defines themaximum time that the electrotherapy device will operate. In a preferredembodiment, two CR2025 3 volt lithium coin cell batteries are connectedin series (6 volts total battery supply). The average current drawn fromthe batteries is approximately 0.9 milliamps when delivering therapeuticpulses of 35 milliamps peak pulse amplitude (350 microsecond pulse widthat 31 hertz frequency) into a simulated human skin load (500 ohmresistor). This current draw compares well to the maximum direct currentdraw for this type of battery, which is typically 3 milliamps. Thetypical battery capacity for the CR2025 is 150 milliampere-hours at acontinuous electrical current draw of 0.2 milliamperes. A draw of 1milliamp should produce somewhat less than 150 hours of battery life.Testing of the electrical circuit using two CR2025 batteries in seriesdemonstrated that the average battery life is about 157 hours. Testswith two CR2016 coin cell batteries (70 milliampere-hour capacity)resulted in an average battery life of 87 hours under the sameconditions. These results are greater than expected based on batterycapacity because the device uses two coin cell batteries for a totalbattery supply of 6 volts, which allows the current draw to beminimized, thereby optimally draining the batteries. The total powerconsumed is P=V*I or approximately P=(6 V)*(0.9 mA)=5.4 milliwatts (mW).A preferred circuit can operate from a single 3 volt battery, but thecurrent consumption must approximately double since the same amount ofpower is needed to deliver the therapeutic pulses. For example, thebattery life of the CR2025 would be expected to be reduced from 157hours to approximately 75 hours. Other battery types, in single ormultiple cell configurations, can be selected with changes to circuitcomponent values made accordingly.

Battery B1 is connected through switch S1 to the pulse generatorcircuit. Switch S1 is operable by the patient and enables the patient toturn on and off the electrotherapy device. Switch S1 is in the closedposition during operation when the patient has turned on theelectrotherapy device. During operation, battery B1 discharges pulsesinto inductor L1. Inductor L1 controls the delivery of current tocapacitor C1 and reduces energy loss to maximize battery efficiency, C1stores the electric charge and accumulates a corresponding voltage untilcommanded to discharge the accumulated voltage to transformer T1,whereupon T1 steps up the voltage for output to the patient in the formof therapeutic output pulses. Microcontroller 15 controls the circuitoperations and comprises FIG. 2 control 6. Microcontrollers aretypically characterized by their operating voltage range, theirelectrical current consumption, their operating speed (clock rate), thenumber of bits used for operations (e.g., 4 bit, 8 bit, 16 bit, etc.),the number of programmable input/output lines, software program storagespace, and integrated special functions (e.g., A/D converters, highcurrent source or sink capability, serial communication ports, etc.).Other factors include cost and availability. 4-bit and 8-bitmicrocontrollers are favored due to their small size, low cost, and lowpower consumption (e.g., Samsung KS51 series and Toshiba TLCS47 series4-bit microcontrollers, and Samsung KS86C series, Toshiba TLCS870 seriesand Microchip 16C5x series 8-bit microcontrollers). A preferredembodiment uses a Microchip 16C54A 8-bit microcontroller.

Switch S1 and microcontroller 15 are connected to transistor Q1, whichtogether with diode D3, and inductor L1 comprise FIG. 2 switchedinductor 7. Microcontroller 15 connects battery B1 to the inductor L1through transistor Q1, which microcontroller 15 operates as a switch.The microcontroller 15 repeatedly opens and closes transistor Q1 to sendbattery discharge pulses to inductor L1. This causes current to flowinto inductor L1 and capacitor C1. Inductor L1 causes this current toincrease at a controlled rate, thereby causing a voltage to developacross capacitor C1 at a controlled rate, thereby reducing energylosses. When transistor Q1 is opened, the current into inductor L1begins to decrease. This causes the voltage across inductor L1 toreverse, thereby causing diode D3 to turn on and complete an electricalcircuit between inductor L1 and capacitor C1. Residual current ininductor L1 is then allowed to flow to capacitor C1, causing its voltageto increase slightly. Once this residual current goes to zero, theinductor L1 voltage is no longer reversed and diode D3 turns off. Thiscauses capacitor C1 to be isolated in the electrical circuit, therebypreserving the voltage stored on it. (Resistors R1 through R5 mayprovide a discharge path for capacitor C1 if any of the switches S2 areclosed. These resistors are chosen to be high values to limit thedischarge current from C1 to acceptably low values.) The value ofinductor L1 is chosen to conserve battery life and provide small sizeand low cost. However, testing has demonstrated that inductor L1 can bereplaced by a smaller, lower cost, low value resistor while stillobtaining the advantage of regulated output while the battery voltagedecreases with use. The drawback of this method is that, while batterylife is enhanced vis-á-vis unregulated output, battery life iscompromised vis-á-vis the switched inductor embodiment due to energylosses in the resistor.

Inductor L1 is connected to capacitor C1, which is chosen typically tobe a high capacitance value to maximize current storage. Current flowingthrough inductor L1 and into capacitor C1 causes voltage to build acrosscapacitor C1 that is proportional to the amount of current deliveredover a particular time period, e.g., the battery discharge time.Microcontroller 15 monitors the charge/voltage built up on the capacitorC1 so it knows when to stop the battery discharge pulses and/or initiatea transformer discharge pulse (therapeutic pulse). Low voltage storagecapacitor C1 is connected to R1, which together with switch array S2 andresistors R2–5 comprise FIG. 2 voltage divider switching network 11.Switch array S2 is manipulated by the patient to select one of a numberof available “intensity” settings. As shown in FIG. 3, switch array S2selects one of a number of resistors in a voltage divider array formedby resistor R1 and resistors R2–5.

R1 of the voltage divider switching network is connected to voltagecomparator 12. Using the voltage comparator, the microcontrollermonitors the voltage across capacitor C1, and continues to allow voltageto build until voltage comparator 12 signals that the voltage hasreached a predetermined voltage value.

The next step is to send a therapeutic pulse from the low voltagestorage capacitor to the transformer. The low voltage storage capacitoris connected to transformer T1, which acts as the output stage 10 ofFIG. 2. Transformer T1 is chosen to have a voltage step-upcharacteristic based on the desired therapeutic output requirements andthe load connected to the electrodes E2 and E2. Once voltage across C1has reached a predetermined value, microcontroller 15 closes eithertransistor Q2 or Q3 to discharge the capacitor into the transformer T1.This sends the voltage to the output stage to be stepped up bytransformer T1. In a preferred embodiment, the transformer has a turnsratio of approximately 20, a low resistance primary winding(approximately 2 ohms), and a high inductance secondary winding(approximately 1 henry). The turns ratio is selected based on themaximum voltage on the storage capacitor at the primary and the desiredmaximum voltage delivered to the skin through the electrodes at thesecondary, e.g., 3 volts at the primary can deliver 3*20=60 volts at thesecondary (other factors such as transformer core saturation must beconsidered). The low resistance primary is needed for reduced powerconsumption. The high inductance secondary is needed to achieve anominally constant current therapeutic output over a range of skinimpedance values. Skin impedance changes with time for a particularpatient, and can be very different between patients. A nominallyconstant current output allows a predictable level of therapeuticcurrent to be delivered regardless of patient skin characteristics,thereby providing better therapeutic value.

Transistors Q2 and Q3 are needed to move electrical current through thetransformer T1 primary winding in one direction or the other, therebycreating positive or negative therapeutic pulses at the electrodes E2and E2. Preferably, the microcontroller alternately operates Q2 and Q3to provide alternately positive and negative pulses to the electrodes.(Alternating operation of Q2 and Q3, together with the center tap 16attachment at the center of the transformer winding, creates a polarityswitching circuit which creates the alternating positive and negativevoltage output from the transformer.) This prevents any iontophoretic orelectropheretic effect on the patient's skin. Alternatively, transformerT1 can be replaced by a standard transformer to create single polaritypulses, or it can be removed and the inductor L1 and capacitor C1 chosento provide the high voltage directly to the electrodes with a differentswitching means to effect different polarity pulses, if required. Theoperation of transistor Q2, Q3 and Q1 may be controlled so that theinductor L1 is always disconnected from the battery when the capacitoris discharging into the transformer. In this manner, current is suppliedto the transformer only from the capacitor and not from the battery.

The circuit can also create a display to the patient. Microcontroller 15is connected to light emitting diodes (LED) D1 and D2 which compriseFIG. 2 dual indicator. In a preferred embodiment, D1 is a green LED thatis flashed at a low duty cycle to conserve battery power and is used toindicate normal operation. D2 is a red LED that is flashed at a fasterrate than D1 and is used to indicate the “low battery” warning.Alternative display methods may be used including liquid crystaldisplay, sound, vibration, etc.

Capacitor C1 can be discharged directly into the skin if certain changesare made to the circuit. Specifically, a diode can be placed in seriesbetween inductor L1 and capacitor C1, which is then chosen to be a highvoltage, high capacitance component, i.e., a standard “boost” regulatorconfiguration. The diode allows a high voltage to be stored on thecapacitor from a lower voltage source. Resistor divider values are thenchosen to suitably divide the peak high voltage down to a value suitablefor the voltage detector. Biphasic pulses can be created using capacitorC1 as an input to a standard H-bridge transistor circuit with suitabletransistors, with the electrodes connected to the middle of the H-bridge(the H-bridge is another form of polarity switching circuit). Thismethod is not preferred because power consumption is relatively high,resulting in low battery life, and the therapeutic output becomesnominally constant voltage instead of the preferred nominally constantcurrent achieved using a transformer or tapped inductor. However, wherethe H-bridge is desirable for other reasons, the battery life may beextended vis-á-vis direct connection to the battery.

The battery discharging circuit is described in connection with its usein an electrotherapy device. However, the method of discharging thebattery at low average current by pulse charging a capacitive storageunit may be employed in various other environments where high currentintermittent loads are powered by a battery. In one example, batteryoperated automobiles and carts with electric motors often stop and idle,then re-load the motor to accelerate the vehicle, thus subjecting thebattery to a high current draw. This high current draw can be reduced bycharging a storage capacitor through a switched inductor while idling,and discharging the capacitor into the motor electrical supply linesupon acceleration or loading of the motor. In another example, largebatteries are used for starting large loads such as coolant motors andstarter motors for engines. These motors typically draw a very largestart-up current when they are turned on. By interposing the circuitdescribed above between the battery and the motor during the start-up,the startup surge may be supplied from the capacitive storage device. Tocharge the capacitive storage device, the starter circuit for the motorwould first charge the capacitive storage device, then start the motorby connecting the motor terminals to the capacitive storage deviceand/or the battery. In this manner, the large start-up current is drawnfrom the battery at a lower discharge rate than it would if the start-upcurrent is drawn directly from the battery into the motor. In anotherexample, a battery powered portable defibrillator uses a battery tocharge a capacitor which is discharged into a patient's chest. The rateat which the capacitor charges can be controlled by placing the circuitbetween the battery and the capacitor. When the defibrillator isoperated, the control 6 then slowly charges the capacitor with a seriesof relatively low current battery discharge pulses. When fully charged,the capacitor is discharged into the patient's chest in the normalfashion. In this manner, the number of defibrillating shocks that can beadministered from a single battery pack is increased. In anotherexample, the battery operated roadside safety beacons which use simpleRC timing circuits can be improved by inclusion of the circuit to lowerthe average current draw on the battery, thereby making the battery lastlonger. Many other battery powered devices which intermittently drawcurrent from the battery may be powered by the circuit to lower theinstantaneous current draw and thereby lengthen battery life.

While the devices and methods have been described in reference to theenvironment in which they were developed, they are merely illustrativeof the principles of the inventions. Other embodiments andconfigurations may be devised without departing from the spirit of theinventions and the scope of the appended claims.

1. A circuit for maintaining consistent pulse output of abattery-powered apparatus to a load, said circuit comprising: a batteryhaving an electrical output; an inductor, connected to the battery, forreceiving the electrical output of the battery; a capacitor, connectedto the output of the inductor, for receiving and storing the electricaloutput of the inductor, the capacitor having an electrical output; anelectrode, connected to the capacitor, for delivering the electricaloutput to the load; and a controller programmed to periodically connectand disconnect the inductor to the battery such that a plurality ofcurrent pulses are generated charging the capacitor, the controller isalso programmed to intermittently connect and disconnect the capacitorto the load causing the capacitor to discharge the electrical output ofthe capacitor to the load, wherein the controller limits the pulsesgenerated by the battery and the time between the pulses generated bythe battery such that the resulting average electrical current draw fromthe battery is within the battery's optimal discharge rate.
 2. Thecircuit of claim 1 further comprising a transformer, connected to thecapacitor, for increasing the electrical output of the capacitor.
 3. Adevice for providing a pulsed electrical stimulus to a nerve in the bodyof a patient, wherein said device is battery operated, said devicecomprising: a battery having an electrical output; an electrode, adaptedfor electrically contacting the body in the vicinity of such nerve to bestimulated, for delivering the electrical output to such nerve; acapacitor, connected to the battery and the electrode, for storing theelectrical output and delivering a electrical stimulus to such nerve; afirst switch, located between the capacitor and the electrode, forperiodically connecting the capacitor to the electrode when thecapacitor is sufficiently charged; a second switch for periodicallyconnecting and disconnecting the battery to and from the capacitor toincrementally charge the capacitor with a plurality of current pulses; acontroller programmed to operate the second switch to periodicallyconnect and disconnect the battery to the capacitor, therebyincrementally transferring charge from the battery to the capacitor bysupplying a plurality of current pulses to the capacitor, saidcontroller being further programmed to periodically connect thecapacitor to the electrode after a desired charge has been accumulatedon the capacitor, to apply a pulsed electrical stimulus to the nerve,wherein the controller limits the pulses generated by the battery andthe time between the pulses generated by the battery such that theresulting average electrical current draw from the battery is within thebattery's optimal discharge rate.
 4. The device of claim 3 furthercomprising a transformer, interposed between the capacitor and theelectrodes, for increasing the electrical output of the capacitor.
 5. Adevice for providing pulsed output from a battery, said devicecomprising: a battery; an electrical output terminal; an electricalstorage component operably and intermittently connected to the outputterminal through a first switch means; a second switch means, locatedbetween the battery and the electrical storage component, for connectingand disconnecting the battery to and from the electrical storagecomponent; control means programmed to intermittently operate the secondswitch thus transferring charge from the battery to the electricalstorage component with a plurality of current pulses generated by theintermittent operation of the second switch, the control means is alsoprogrammed to intermittently connect the electrical storage component tothe output terminal after a desired charge has been accumulated on theelectrical storage component, wherein the control means limits thepulses generated by the battery and the time between the pulsesgenerated by the battery such that the resulting average electricalcurrent draw from the battery is within the battery's optimal dischargerate.
 6. An apparatus for regulating the output of a battery to supply aload in a battery powered device comprising: a switch; an inductorhaving an output; a capacitor being connected to the inductor andconnected to the load through the switch, the conductor stores theoutput from the inductor and delivers the output to the load; acontroller programmed to operate the inductor by intermittentlyconnecting and disconnecting the inductor to the battery such that thebattery outputs a battery discharge pulse to the inductor, and theinductor transmitting a plurality of current pulses to the capacitor tobuild a charge of current on the capacitor, the controllerintermittently connecting the capacitor to the load causing thecapacitor to discharge the current pulse to the load, wherein thecontroller limits the pulses generated by the battery and the timebetween the pulses generated by the battery such that the resultingaverage electrical current draw from the battery is within the battery'soptimal discharge rate.
 7. A method for controlling the discharge of abattery to a load, said method comprising: providing a switchedinductor, said switched inductor having an inductor input switch with aninput adapted to be connected to the battery, said switched inductorhaving a current output; providing a capacitor adapted to be placed incircuit with the output of the inductor and collect the current outputfrom the inductor; providing a capacitor output switch in circuit withthe capacitor, said capacitor output switch adapted to be connected tothe load; and providing a control module to control the operation of theinductor input switch and the capacitor output switch; programming thecontrol module to: monitor the voltage on the capacitor; periodicallyoperate the inductor input switch to generate a plurality of currentpulses from the battery to the capacitor; allow the current pulses togenerate a build-up of voltage on the capacitor to a predeterminedvoltage; and operate the capacitor output switch to connect thecapacitor to the load thereby discharging the voltage on the capacitor,wherein the control module limits the pulses generated by the batteryand the time between the pulses generated by the battery such that theresulting average electrical current draw from the battery is within thebattery's optimal discharge rate.